Lore:
This is a world where Teddy Roosevelt wasn't president.
Seriously okay hear me out-- McKinley doesn't get shot to death, so Teddy doesn't become president. That knocks the knees out of the Progressive Movement so that they never gain power. So, from 1900-1929, US workers suffer in Gilded Age conditions. The Communist Party becomes real popular and when the Great Depression hits, launches a revolution, toppling the government (I made another map with this scenario).
Afterward, the USSR and Communist America are allies in WW2, but America is too distracted by its revolution and Japan to help the USSR beat Nazi Germany. Eventually the USSR develops nukes and uses them to defeat Nazi Germany, conquering Europe. Communist America and Russia are friends for a little bit longer before turning on each other (Mao and Khrushchev did) and plunge into WW3, nuking major cities across the planet. Eventually they make peace and decide to split the world between them.
I go into more depth about it, and talk about what happens next and if they collapse here:
Oh, and also Earl Browder (CPUSA chief) is Big Brother. He's got the mustache and everything.